If your yields have been good I wouldn't worry. You must be doing something right. When I used to do tomatoes in large pots I only put one in each. Gave up when the squirrels ate more than we did. Might try them again next year after I get my garden/green house.

Depends on how big the pot is.
What was the reasoning behind only having one plant?

The only reason I would think they might say that is if it was overcrowded. If the plant has the space then I see no reason not to put more than one plant in.

Probably the best way to see what the difference might be is to plant only one plant in a container and compare the yield to others grown in groups.

My neighbor plants Tumbling Toms, one to a 12" hanging basket and they get to be a good sized plant. I've read recommendations to put 2 or even 3 to a basket - definitely not necessary. Last year I grew just one Tumbling Tom in a 5 gallon nursery pot - way more room than a 12" basket. The plant was massive and had tons of fruit, and at the end of the year that pot was all rootball. But even with all that soil and room for the roots I had to water it deeply every day once it got so big and the weather was hotter.

I thought there was some study about a tomato having a certain amount of cubic footage to grow properly but I can't remember where I might have seen that. It certainly would depend on the variety, I think, as some plants are quite small and more suitable to container growing.